1. Field of the Invention
The present invention relates an acceleration sensor including a piezoelectric element.
2. Description of the Related Art
FIG. 11A shows a conventional acceleration sensor including a piezoelectric ceramic. The acceleration sensor includes a bimorph piezoelectric element 20 supported at one end thereof. The piezoelectric element 20 is made by bonding two piezoelectric ceramic layers 21 and 22 together and by connecting the two piezoelectric ceramic layers 21 and 22 in series. Also included are major surface electrodes 23 and 24, an interlayer electrode 25, a support member 26, and electrode leads 27 and 28. FIG. 11B is a circuit diagram of the acceleration sensor. The polarization directions P of the layers 21 and 22 are opposite to one another in the direction of thickness such that voltages generated in response to the application of acceleration G are summed.
When the acceleration sensor is subjected to a temperature change, voltages are generated in each of the layers 21 and 22 because of the pyroelectric effect. Voltages generated in the layers 21 and 22 in response to a temperature drop are shown in FIG. 11A. Since the directions of voltages of the two layers 21 and 22 are opposite, the voltages are maintained without being canceled. The voltages remain unchanged even if the electrodes on both ends of the sensor are short circuited to each other. The polarities of the voltages are inverted from those during polarization. The voltages depolarize the layers. In particular, a surface mount type acceleration sensor, which undergoes reflow mounting, is subjected to a quick temperature drop. A large pyroelectric voltage is generated which depolarizes the layers and causes a drop in sensitivity of the sensor.
FIG. 12A shows a bimorph piezoelectric element 30 including two piezoelectric ceramic layers 31 and 32 connected in parallel. FIG. 12B is a circuit diagram of the piezoelectric element 30. The two layers are polarized in the same polarization direction P such that the polarities of the voltages are the same under the application of acceleration G. The electrodes 33 and 34 on the top and bottom surfaces are connected to a pickup electrode 35, and the interlayer electrode 36 is connected to the other pickup electrode 37.
Voltage is generated in each of the layers 31 and 32 in response to a temperature change because of the pyroelectric effect. The voltage distributions are directed such that the generated voltages cancel each other in a parallel connection. The pyroelectric voltages thus cancel each other in the sensor. As a result, no voltage is generated in each of the layers 31 and 32. The sensor including the two layers 31 and 32 connected in parallel has a voltage sensitivity lower than the serially connected type sensor shown in FIG. 11A. If an insulation resistance is reduced in any portion of the layer, the sensitivity of the entire piezoelectric element is reduced. Increasing the thickness of the layer increases the sensitivity. If there is a possibility that the insulation resistance is reduced, the thickness of the layers cannot be reduced. Thus, it is difficult to increase the sensitivity.
The serial connection of the layers provides the advantage of high voltage sensitivity, while suffering from polarization drop due to the pyroelectric voltage. The parallel connection of the layers provides the advantage of preventing the polarization drop responsive to the pyroelectric voltage, but suffers from low voltage sensitivity. Furthermore, increasing the sensitivity is difficult because the thickness of the layers cannot be reduced because of the possibility of decreased insulation resistance.
To overcome the problems described above, preferred embodiments of the present invention provide an acceleration sensor having a high voltage sensitivity, and which does not depolarize due to a pyroelectric voltage.
In a first preferred embodiment of the present invention, an acceleration sensor includes a piezoelectric element and a support member for supporting the piezoelectric element at both longitudinal ends thereof. The piezoelectric element includes a laminate of three piezoelectric layers. The intermediate piezoelectric layer of the three piezoelectric layer laminate is a dummy layer which generates no charge when acceleration is applied thereto. Each of the two outer piezoelectric layers includes four longitudinally aligned regions separated at two borders and one central border where stress is inverted when the acceleration is applied. The two outer piezoelectric layers are polarized in the direction of thickness of the piezoelectric element such that cells adjacent to each other in the longitudinal direction are inverted in the polarization direction and such that cells corresponding to each other in the direction of thickness are polarized in the same polarization direction. Electrodes are arranged on the top and bottom major surfaces of the piezoelectric element and between the piezoelectric layers such that two cells on one side of the central border are connected in parallel, and the other two cells on the other side of the central border are connected in parallel in each of the two outer piezoelectric layers, and such that the two parallel-connected cells on the one side are connected in series to the other two parallel-connected cells on the other side. At least one of the electrodes on the top and bottom major surfaces and between the layers extends to different end surfaces in the longitudinal direction of the piezoelectric element.
A voltage is generated in each piezoelectric layer in response to a temperature change applied to the acceleration sensor. Two cells on one side of the central border which are disposed towards the center of the longitudinal length of the element are now discussed. Since the polarization directions of the two cells are opposite, the polarity of the pyroelectric voltage is opposite from the polarization direction. The pyroelectric voltage extends in a direction which depolarizes the cell. Since the two cells are connected in parallel, charges generated through pyroelectricity immediately cancel each other in the region of the two cells. Because of this, no depolarization occurs, and no sensitivity drop is caused.
Since the two cells on the one side of the central border and the two cells on the other side of the central border are serially connected, the voltage sensitivity is greater than that of the parallel connection type sensor.
Even if an insulation resistance is reduced between electrodes of the two cells on the one side of the central border, the two cells in the same piezoelectric layer on the other side of the central border are not affected. Since the piezoelectric layer is isolated from the other piezoelectric layer by the dummy layer, the other piezoelectric layer is also free from the reduced insulation resistance in the two cells. The effect of the reduced insulation resistance on the characteristics of the entire sensor is thus prevented. This produces a humidity resistant sensor. Given the same humidity level, a thinner piezoelectric layer operates effectively, and a more sensitive sensor is provided.
Since a detector cell is provided near the external surface of the element where a change in stress is greater than in a neutral plane, the sensitivity of the sensor is greatly increased. The thickness of the detector cell is reduced while the thickness of the sensor, which affects the durability of the sensor, is maintained. A high sensitivity sensor is thus provided.
In a second preferred of the present invention, an acceleration sensor includes a piezoelectric element and a support member for supporting the piezoelectric element at both longitudinal ends thereof. The piezoelectric element includes a laminate of two piezoelectric layers. Each of the two piezoelectric layers includes four longitudinally aligned regions separated at two borders and one central border where stress is inverted in the longitudinal direction of the piezoelectric element when the acceleration is applied. The two outer piezoelectric layers are polarized in the direction of thickness of the piezoelectric element such that cells adjacent to each other in the longitudinal direction are inverted in polarization direction and such that cells corresponding to each other in the direction of thickness are polarized in the same polarization direction. Electrodes are arranged on the top and bottom major surfaces of the piezoelectric element such that two cells on one side of the central border are connected in parallel, and the other two cells on the other side of the central border are connected in parallel in each of the two outer piezoelectric layers, and such that the two parallel-connected cells on the one side are connected in series to the other two parallel-connected cells on the other side. At least one of electrodes on the top and bottom major surfaces and between the layers extends to different end surfaces in the longitudinal direction of the piezoelectric element.
As in the acceleration sensor of the first preferred embodiment of the present invention, even if a pyroelectric voltage is generated in response to a temperature change in the acceleration sensor of the second preferred embodiment of the present invention, the charges generated in response to the pyroelectric effect immediately cancel each other within the region of the two cells because the two cells on the same side of the central border are connected in parallel. Thus, the sensitivity of the sensor is not reduced by depolarization.
Since the two cells on the one side of the central border and the two cells on the other side of the central border are serially connected, the voltage sensitivity is greater than that of the parallel connection type sensor.
Even if an insulation resistance is reduced between electrodes of the two cells on the one side of the central border, the two cells in the same piezoelectric layer on the other side of the central border are not affected.
Since the number of piezoelectric layers and the number of electrodes are reduced, manufacturing costs are reduced.